Patent Document 1 discloses a thin film transistor in which a first gate electrode is formed on a substrate, a first gate insulating layer is formed so as to cover the first gate electrode, a semiconductor layer formed using an oxide semiconductor is formed over the first gate insulating layer, a second gate insulating layer is formed over the semiconductor layer, a second gate electrode is formed over the second gate insulating layer, and a drain electrode and a source electrode which are connected to the semiconductor layer are formed, wherein the thickness of the second gate electrode is equal to or larger than that of the first gate electrode is described (claim 1). In Patent Document 1, the above-described thin film transistor can be used for a driving field-effect transistor of a liquid crystal display or an organic EL display (144th paragraph).